JP-2006-299945A shows an internal combustion engine where each cylinder is provided with two fuel injectors for two respective intake ports, so that fuel spray is atomized and a quantity of fuel adhering on an inner wall surface of an intake port is reduced.
JP-8-338285A (U.S. Pat. No. 5,730,111) shows an air-fuel ratio control system where an air-fuel ratio (fuel injection quantity) is controlled with respect to each cylinder based on outputs of an air-fuel ratio sensor disposed at a confluent portion of exhaust gas discharged from each cylinder.
A fuel injection quantity may have an error (a deviation in an actual fuel injection quantity from a command fuel injection quantity) due to an individual manufacturing tolerance or aging of a fuel injector. If the air-fuel ratio control shown in JP-8-338285A is applied to an internal combustion engine shown in JP-2006-299945A, an error of total fuel injection quantity can be corrected. However, an individual error of each fuel injector can not be corrected. Thus, in a case that a fuel injection ratio between two fuel injectors is changed in order to reduce emission and improve fuel economy, the fuel injection ratio between two fuel injectors can not be correctly controlled.